1. Field of the Invention
The present invention relates generally to the field of electric power, and more particularly to devices and methods for detecting the islanding operation of a static power source connected to a utility grid.
2. Description of the Related Art
FIG. 1 is a block diagram that shows an example of a static power source (SPC) 10 connected to and in parallel operation with a grid 12 when a breaker Brk 1 is closed. The electrical loads LD1 and LD2, shown hanging at local power lines 14, can obtain power from both the SPC and/or the grid when the breaker Brk 1 is closed. However, when the grid voltage Va-g, Vb-g, Vc-g is lost, for example, by opening the breaker Brk 1, the loads LD1 and LD2 can still obtain electrical power Va-spc, Vb-spc, Vc-spc from the SPC 10 through the local power lines 14. That situation, in which the operation of a local power source, such as the SPC 10 in FIG. 1, continues to power the local power lines and load after the grid voltage Va-g, Vb-g, Vc-g is disconnected is called islanding operation.
Islanding operation is not desirable due to safety concerns. Detecting the islanding operation of a local power source, such as SPC 10, and isolating the local power source from the local power lines 14 after the grid 12 is lost is typically made mandatory by regulation. It is mandatory to have an anti-islanding feature on inverters, or utilities will not allow such inverters to be connected to the utility grid. In addition to safety issues, islanding also has performance implications. Current active methods of addressing the islanding problem include injecting harmonics which compromise performance. There are also passive methods which have non-detection zones, so anti-islanding is not guaranteed for all conditions. There is a separate issue of multi-unit operations, which also poses a problem for harmonic injection because harmonics can cancel each other.
Existing methods use harmonics or frequency pulse injection, or an explicit positive voltage or frequency feedback technique to detect the grid loss. These methods have serious drawbacks. Such injected signals can interfere with the grid or the load. The injected signals can sometimes cause resonance. The injected signals can also cancel each other, rendering the technique useless when two or more SPCs are connected together. The harmonics introduced to the grid affects the grid quality. The explicit frequency or voltage drifting technique is based on grid frequency/voltage change, and a positive feedback is used to drift the frequency/voltage away. This method cannot guarantee that islanding can be detected if there are no voltage or frequency changes when islanding occurs. At a special operation point (i.e., ideal islanding condition), the SPC's output power and power factor match the load's power and power factor on the local grid, thus there will be no frequency and voltage changes when islanding occurs, causing the above described explicit frequency or voltage drifting technique to fail.
Not only may there be a non-detection point (NDP) as it is described in the above ideal islanding condition, there may also be a non-detection zone (NDZ) associated with the above described method. When the SPC's power and power factor are very close to the load's power and power factor, the frequency and voltage change will be very small when islanding occurs. In order to detect these small changes and build up the drift, a larger gain will have to be used for the positive feedback of the explicit frequency or voltage drifting technique. However, a larger gain for positive feedback can cause the system to become unstable, while a smaller gain can potentially make the system have a larger NDZ in detecting islanding.